1. Field of the Invention
The present invention is directed to a method and an apparatus for removing spurious data ("data outliers") produced by externally-originating electromagnetic interference (EMI) from a sensed signal supplied to an implanted medical device, such as a cardiac stimulator.
2. Description of the Prior Art
As is well-known, implanted medical devices, such as cardiac stimulators (pacemakers, defibrillators, etc.) commonly employ an electrode lead, extending from an implanted electronic unit, in order to sense electrical activity in the subject so as to control the electrotherapy (pacing, defibrillation, antitachycardia routine, etc.) administered by the implanted stimulator to the subject. The electrode lead is typically plugged at one end into the implanted electronic unit, and has an opposite end located adjacent cardiac tissue, or in the bloodstream, or at some other appropriate location depending on the type of electrical activity or physiological parameter being sensed. Since this electrode lead contains one or more conductors extending from the implanted device to the sensor or electrode at the tip of the lead, the lead itself acts as an antenna, and thus is susceptible to receiving signals, such as electromagnetic interference (EMI) originating from external sources. Such interference (noise) corrupts the "true" waveform originating from the sensed electrical activity, and therefore when the corrupted sensed signal is analyzed within the implanted device, in order to produce a control signal for the therapy administration, the analysis may be falsified because the noise may produce spurious data points (data outliers) in the analyzed data.
One such type of external interference which is currently under review to determine its impact on implantable medical devices is that produced by cellular telephones. Cellular telephones transmit voice messages by emitting signals from an antenna using radio waves at frequencies between 824 and 894 MHZ. In the system currently in widespread use in the United States, digital cellular hand-held phones employ a maximum of 0.6 watts of power to transmit messages to a cellular transmitter tower. The power level used by the cellular telephone fluctuates throughout the duration of a call. At a large distance from the tower, the hand-held instrument may use the full 0.6 watts. If the caller is closer to the tower, the hand-held telephone may only require 0.05 watts, for example, to effectively transmit the signal. The number of hand-held telephones being used on a system at any given time also affects the transmission power.
Cellular telephones can transmit either analog or digital voice messages, dependent on the type of hand-held instrument and the type of service available. In analog systems, messages are transmitted by modulating or varying either the amplitude of the signal or the frequency of the signal. In digital systems, messages are transmitted in a series of rapid bursts or pulses. An advantage of digital transmission, which is expected to be increasingly employed, is that it increases channel capacity by allowing several users to transmit messages at the same frequency at the same time.
Two types of digital technology currently in use in the United States are relevant to the issue of implanted stimulator/telephone interaction. These are Code Division Multiple Access (CDMA) and Time Divisional Multiple Access (TDMA). In CDMA, messages are transmitted as various sequences of ones and zeros with a special code attached thereto, so that only the intended receiver is able to decode the message. In TDMA, data are transmitted in bursts by turning the signal on and off fifty times per second, causing the signal to have the appearance of a pulsed signal. This technique is therefore sometimes described as "pulse-modulated" RF radiation.
The current standard for use in Europe is the Global Standard For Mobile Communications (GSM) technology. GSM technology uses TDMA technology, and operates in a frequency range between 890 and 960 MHZ. GSM technology uses a 217 Hz pulse rate. The power generated by a GSM portable telephone ranges from 0.02 watts to 2 watts. Generally, the power generated by an instrument designed for use in Europe is higher than an instrument designed for use in the United States, because of the larger distance which the European instrument must transmit its signal in order to reach a base station. The density of base stations in Europe is lower than in the United States.
Because of the widespread use of cellular telephones, a pacemaker wearer will almost certainly randomly enter into and leave the transmission fields of a number of such cellular telephones during the course of a day. Of course, if the pacemaker wearer himself or herself uses such a cellular telephone, the potential is greatest for the signal from that telephone interfering with sensed signals used to control the operation of the pacemaker. It is, of course, well-known in pacemaker technology to filter internally obtained, sensed signals in order to remove noise therefrom, such as noise produced by respiration and other internal sources. Conventional noise removal techniques generally employ some type of filtering, the intention being to remove the noise contribution as much as possible from the overall signal, thereby leaving a filtered signal which constitutes a "clean" representation of the sensed activity or the sensed parameter.
In the case of interference produced by a cellular telephone, particularly a cellular telephone employing GSM technology, the pulses of the GSM signal produce relatively pronounced spikes which are (or may be) superimposed on the internally sensed signal. The amplitude of these spikes usually significantly exceeds the expected amplitude of any portion of the sensed signal. Moreover, the spikes are of extremely short duration. Attempting to filter out the effects of such cellular telephone interference by conventional filtering (such as low-pass filtering, bandpass filtering or high-pass filtering) would unavoidably also remove a large amount of the sensed signal itself, thereby removing an unacceptably high amount of data. Since the data content of the sensed signal is used to control the implanted device, this means that the control of the device will inherently be much less accurate. A device of this type is described, for example, in European Application 0 713 714, which also teaches identification of a noise threshold which, when exceeded, causes one or more remedial measures to be selected.
Median filtering is generally known in the signal processing field as a type of filter which determines a median signal value for a finite neighborhood around each input data point. Examples of median filters described for general signal processing use are found in U.S. Pat. No. 5,114,568 and U.S. Pat. No. 5,138,567. In the field of medical technology, the use of median filtering is generally known for processing ECG signals, as described in U.S. Pat. No. 5,343,870. The use of a median filter for assisting in analyzing an incoming atrial signal to identify the presence of atrial fibrillation is described in U.S. Pat. No. 5,527,344.